This invention relates generally to hydrostatic transmissions and, more particularly, relates to a novel pump and center section for use in hydrostatic transmissions.
Hydrostatic transmissions (“HSTs”), including integrated hydrostatic transmissions (“IHTs”), are well known in the art and are more fully described in, among others, U.S. Pat. No. 5,314,387, which is incorporated herein by reference in its entirety. Generally, an HST includes a center section on which is mounted a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons which are in fluid communication through pumping and intake porting formed in the center section. Rotation of the hydraulic pump by an engine creates an axial motion of the pump pistons against a swash plate which forces a hydraulic fluid through the pumping porting to the hydraulic motor which moves the motor pistons in an axial direction. The axial movement of the motor pistons causes the hydraulic motor to rotate as the motor pistons bear against a thrust bearing. As the hydraulic motor rotates, the hydraulic fluid is returned to the hydraulic pump through the intake porting. In this manner, the rotation of the hydraulic pump is translated to the hydraulic motor which is then used to drive vehicle axles of a riding lawn mower, small tractor and the like.
For use in providing smoother and quieter movement of the hydraulic pump as it rotates on the center section, it is known to provide grooves in the lands of the center section at the ends of the port openings. More specifically, the purpose of the grooves is to smooth and quiet the movement of the hydraulic fluid as the pump cylinder block piston port openings move relative to the center section port openings. A typical groove having a generally uniform groove width may be seen in FIG. 17 of U.S. Pat. No. 6,322,474, which is incorporated herein by reference in its entirety. This typical groove design, however, has not been found to provide sufficient smoothness and quietness when used in connection with the operation of a 10 cubic centimeter (“cc”) per revolution, seven-piston hydraulic pump.
A further exemplary groove having a “V” shape which is formed in a porting plate can be seen in FIGS. 3 and 4 of U.S. Pat. No. 3,641,765. While the '765 patent describes the “V” shaped groove as having at its deepest point a depth of the order of one-seventh of the thickness of a porting plate, the '765 patent fails to provide a concrete solution with respect to the problem of how to provide a 10 cc per revolution, seven-piston hydraulic pump and center section combination with a smoother and quieter mode of operation. Rather, the '765 patent ambiguously states that an “optimum groove configuration can best be found by trial and error for any given design of pump, choice of working pressure, etc.”